1. Field of the Invention
The present invention relates to a driving technique for a motor.
2. Description of the Related Art
As a cooling fan motor, a spindle motor for rotating an optical or a magnetic disk, or a capstan motor employed in a tape recorder apparatus, a brushless DC motor is employed. A typical brushless motor includes a rotor having a permanent magnet and a stator having a coil. With such a brushless DC motor, the electric current to be supplied to the coil is controlled so as to excite the coil, thereby rotationally driving the brushless DC motor.
Techniques for detecting the rotational position of the rotor of a brushless DC motor are known, examples of which include techniques using Hall effect sensors or techniques for a sensorless driving operation using back electromotive force which occurs when phase transmission occurs. The Hall effect sensors generate a pair of Hall signals (which will be collectively referred to simply as the “Hall signal”) that change in a complementary manner according to the position of the rotor of the motor.
A driving circuit for a motor switches the polarity of the driving voltage to be supplied to the coil, based upon the timing at which the Hall signal or the back electromotive force is detected. In a case in which a Hall signal having a sinusoidal wave waveform or a trapezoidal wave waveform is obtained according to changes in the polarity of the rotor, the driving circuit for the motor gradually changes the driving voltage and the polarity to be applied to the coil, using the change in the voltage in the Hall signal before and after the switching timing, thereby reducing the motor driving noise. Such an arrangement is also referred to as a “soft switch”.
With regard to such a motor which is capable of detecting the rotor position using Hall effect sensors, a technique is known in which Hall effect sensors are integrated on a single circuit on which a motor driving IC is mounted, instead of employing Hall effect sensors in the form of external components, thereby reducing the number of components mounted on a substrate. However, where such Hall effect sensor components are integrated on the driving circuit using the silicon process, highly magnetically sensitive material cannot be employed, unlike an arrangement employing Hall effect sensors in the form of external components. This leads to a problem of reduced magnetic sensitivity. In order to compensate for this problem, the signal to be used is amplified on the integrated circuit. However, such an arrangement also amplifies the offset component of the signal output in a complementary manner. If signal processing is performed to cancel out the offset component, the difference in the magnetic field before and after the switching timing, which is to be detected by the Hall effect sensor, cannot be detected, and thus the soft switching technique using this difference cannot be used in such an arrangement. Similarly, in a case in which a sensorless technique using back electromotive force is employed, only the timing at which the back electromotive force is generated is detected, and thus, like the aforementioned arrangement employing Hall effect sensors, the soft switching technique according to the change in the magnetic field cannot be used in such an arrangement.